This invention relates to a method and apparatus for measuring the chemical content of solid core samples. More particularly, the core sample analysis of this invention can be performed immediately without requiring laboratory analysis and calculations, or destructive techniques.
The present invention is described herein in the context of a method and apparatus for measuring the kerogen content of oil shale, although the method and apparatus can be used to measure the content of other elements, compounds, or groups of compounds in other solid core samples.
Vast untapped reserves of oil entrapped in oil shale exist in this country as well as other locations throughout the world. Increased attention is being devoted to the exploitation of oil shale because of the current need for new energy sources.
The term "oil shale" as used in the industry is in fact a misnomer, because it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposits interspersed with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the carbonaceous liquid product is called "shale oil."
Oil shale deposits occur in generally horizontal beds, and within a given bed there are an extremely large number of generally horizontal deposition layers containing kerogen known as "varves." The varves typically are non-uniformly dispersed throughout a given bed. In the higher grade oil shales, the varves are commonly cemented together into relatively thick, compact layers. The lower grade oil shales have much thinner varves spaced apart through the bed. For example, a core sample taken from a typical bed may vary from thick, solid oil shale sections to layered sections having the appearance of poker chips. In the core, small layers or nodules of other minerals and rock materials are sometimes found interspersed between the varves containing kerogen. Typical of these other minerals are nahcolite, dawsonite, other salines, dolomite, mudstone, sandstone, tuffs, analcite and bentonite.
Techniques for preparing shale oil for retorting generally comprise explosively expanding a subterranean oil shale formation to form a fragmented, permeable mass of oil shale particles. Shale oil then may be recovered from the particles by in situ retorting techniques, or by retorting in surface retorts, for example.
Prior to preparing the oil shale for retorting, the formation is explored to determine the locations within the formation containing the highest grades of oil shale. Core samples are taken from the formation and subjected to laboratory analysis to determine the kerogen content of the sample. One such analytical technique is the "Fischer assay" in which a sample customarily weighing 100 grams and representing one foot of core is subjected to controlled laboratory analysis involving grinding the sample into small particles which are placed in a sealed vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton," referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as the Fischer analysis.
Such analytical techniques are generally done in laboratories far from the drilling site. This causes a considerable delay before analytical results are available to field personnel conducting the exploration tests. Thus, immediate field decisions on the progress of the exploration program cannot be based on accurate analyses of core samples.
The present invention provides a method and apparatus for rapidly determining the chemical composition of a core sample. For example, the kerogen content of oil shale can be rapidly measured without requiring calculations or destructive laboratory techniques characteristic of the Fischer assay and other known methods for measuring kerogen content. The invention makes it possible to accurately analyze a core sample in the field and still have the core sample available for other purposes, such as corroborating laboratory measurements to be conducted later at a more convenient time. Thus, field decisions on the progress of the exploration program can be made immediately, rather than waiting for several days, which is a common delay for current kerogen analysis techniques, for example.